A slow-cooled blast furnace slag applicable as a coarse aggregate for concrete or a road subbase course material, i.e., a crystalline blast furnace slag, has conventionally been manufactured as follows:
(1) Pouring a molten blast furnace slag discharged from a blast furnace through a slag runner into a pit, and slowly cooling the molten blast furnace slag in the pit in contact with open air while applying an appropriate water sprinkling; PA1 (2) Pouring a molten blast furnace slag discharged from a blast furnace through a slag runner into a molten slag ladle mounted on a carriage, transferring the carriage to a slag treating yard having a slope, pouring the molten blast furnace slag from the molten slag ladle onto the slope of the slag treating yard, and slowly cooling the molten blast furnace slag poured onto the slope in contact with open air while applying an appropriate water sprinkling. PA1 (a) When employing the crystalline blast furnace slag as a course aggregate for concrete, the high porosity of the crystalline blast furnace slag leads to a lower workability, and cement paste which fills numerous pores of the slag results in larger water and cement consumptions. PA1 (b) Sulfur as a simple substance (S.sup.0) is produced in the molten blast furnace slag during solidification thereof. The molten blast furnace slag contains, on the other hand, divalent sulfur (S.sup.2-). Therefore, when the crystalline blast furnace slag containing simple-substance sulfur and divalent sulfur is used as a road subbase course material, the high porosity of the crystalline blast furnace slag causes production of polysulfide ions through reaction of water with simple-substance sulfur and divalent sulfur, thus resulting in the production of yellow leaching liquid, which is not desirable in environmental control, from the crystalline blast furnace slag. Because the crystalline blast furnace slag producing yellow leaching liquid is not desirable as a road subbase course material, the crystalline blast furnace slag can be used as a road subbase course material only after the completion of the reaction of water with simple-substance sulfur and divalent sulfur through positive production of yellow leaching liquid effected by the contact with water for a period of from three to six months in an appropriate treating yard. PA1 (c) Since the crystalline blast furnace slag is manufactured by treating a large quantity of a molten blast furnace slag at a time, uniform cooling conditions cannot be given to the molten blast furnace slag at all places in a pit or on a slope of the slag treating yard. The crystalline blast furnace slag manufactured as mentioned above involves consequently large variations in the quality. PA1 (i) The molten slag is supplied onto the outer surface of the rotary drum in open air. It is therefore inevitable that air is entangled into the molten slag during supply of the molten slag onto the outer surface of the rotary drum. In addition, the molten slag supplied onto the outer surface of the rotary drum contacts with air over a large area. As a result, the molten slag supplied onto the outer surface of the rotary drum is oxidized by air, thus causing production therein of such gases as sulfur dioxide, carbon monoxide and carbon dioxide as mentioned above. Since there is no restricting force imposed on the produced gases, this results in a solidified slag with a high porosity. The solidified slag thus obtained contains therein simple-substance sulfur and divalent sulfur which produce yellow leaching liquid through reaction with water. When employing an ejecting nozzle, the molten slag supplied onto the outer surface of the rotary drum is subjected to oxidation by air accelerated by the impact of compressed air from the ejecting nozzle. PA1 (ii) Since the molten slag supplied onto the outer surface of the rotary drum forms a layer having a certain thickness on the outer surface of the rotary drum, cooling is started mainly from the portion in contact with the outer surface of the rotary drum. Therefore, a high cooling rate cannot be obtained over the entire molten slag supplied onto the outer surface of the rotary drum, thus making it difficult to prevent gas bubbles from developing. PA1 (iii) The molten slag is continuously supplied onto the outer surface of the rotary drum through the slag runner. It is therefore difficult to supply the molten slag into a uniform thickness on the outer surface of the rotary drum. As a result, a solidified slag in uniform cooling conditions cannot be obtained.
However, a molten blast furnace slag contains sulfur, carbon, nitrogen and hydrogen which are gasified by oxidation. On the other hand, the molten blast furnace slag discharged from a blast furnace entraps air during pouring from a slag runner into a pit or onto a slope of the slag treating yard. In the course of solidification of the molten blast furnace slag, therefore, sulfur in the molten blast furnace slag is, for example, gasified into sulfur dioxide through oxidation, a part of which is released to open air, and carbon in the molten blast furnace slag is gasified into carbon monoxide or carbon dioxide, a part of which is released to open air. On the other hand, since the molten blast furnace slag is cooled at a low cooling rate through solidification thereof, numerous developed large bubbles of gases produced through oxidation such as sulfur dioxide gas, carbon monoxide gas and carbon dioxide gas as mentioned above are entangled in the solidified blast furnace slag during solidification thereof. Therefore, the conventional slow-cooled blast furnace slag, i.e., the conventional crystalline blast furnace slag, which has thus a high porosity, involves the following problems when used as a coarse aggregate for concrete or a road subbase course material:
Under such circumstances, a method and an apparatus for treating a molten slag has been proposed in Japanese Pat. Provisional Publication No. 102,292/78, dated Sept. 6, 1978, which comprises:
continuously supplying a molten slag from above a rotating rotary drum through a slag runner onto the outer surface of said rotary drum, and on the other hand, blowing a cooling medium onto the inner surface of said rotary drum to cool the barrel of said rotary drum; cooling and solidifying said molten slag through contact with the outer surface of said rotary drum thus cooled, and simultaneously, cooling and solidifying, as required, said molten slag supplied onto the outer surface of said rotary drum by air blown from an ejecting nozzle arranged near said rotary drum; and, scraping out said solidified slag from the outer surface of said rotary drum by a scraper (hereinafter referred to as the "prior art").
The prior art, which has an advantage of not requiring a large space as in the above-mentioned conventional method for manufacturing a crystalline blast furnace slag comprising slowly cooling a molten blast furnace slag through contact with open air in a pit or on a slope of the slag treating yard, involves the following problems: